The present invention relates to a reactor core and a control rod system of a boiling water type nuclear reactor for power generation.
A core of an advanced boiling water type nuclear reactor (hereunder, referred to as ABWR) which is the newest of the boiling water type nuclear reactors used for power generation is constructed of fuel assemblies 2 arranged in a lattice form and control rods 4 between the fuel assemblies 2, as shown in FIG. 8. The fuel assemblies 2 each are constructed of a plurality of fuel rods and a channel box 1. The control rods 4 are used for reactivity control at time of operation, emergency shutdown of the reactor (scram) and reactor shutdown. The control rods 4 are inserted between channel boxes which are outer walls of the above-mentioned fuel assemblies 2 by a driving mechanism arranged at a lower portion of the nuclear reactor pressure vessel.
FIGS. 9 and 10 show a construction of a control rod 4. The control rod 4 is formed of a body and control rod blades 3 extending from the body in 4 directions. Each control rod blade 3 has neutron absorbers 8 inserted therein, and the control rod blades 3 are inserted inside the core to absorb excessive neutrons, thereby to control excessive reactivity.
JP A 6-174874 discloses a technique that a fuel assembly is large-sized while maintaining thermal margin and reactor shutdown margin, whereby the fuel assemblies loaded in the core are made large in size and the number of the fuel assemblies is reduced, thereby to effect labor-saving for fuel exchange.
As shown in FIG. 11B, in a large-sized fuel assembly core, positions of the control rods 4 are the same as in the conventional lattice core, but the control rods are rotated by 45xc2x0 , and the large-sized fuel assemblies are arranged inside the control rods. Therefore, the large-sized fuel assembly corresponds to 2 conventional fuel assemblies. Further, form a point of view of securing a reactor shutdown margin, the blades 3 of the control rod are made large-sized, and arranged between the channel boxes 1 on the diagonal of the fuel assemblies.
However, the above-mentioned conventional technique has the following problems which need to be solved.
That is, although the number of fuel assemblies is reduced by making the fuel assemblies into a large size, the number of control rods is nearly equal to that in a conventional plant. In order to sufficiently secure a shutdown margin of the large-sized fuel assemblies, the blade length of the control rod is increased, whereby a cost of a control rod is raised, so that the plant as whole becomes high in cost.
Further, in order to advance making higher the burning degree and the saving of Uranium, an amount of loaded fuel and the number of Gd fuel rods increase and a reactor shutdown margin decreases.
An object of the present invention is to provide a boiling water type nuclear reactor core which is provided with control rods enabling to secure sufficient control rod worth without widely changing equipment and making higher a manufacturing cost and an operation method thereof.
According to the present invention, in order to achieve the above object, in a boiling water type nuclear reactor core in which a plurality of fuel assemblies each enclosed in a channel box are loaded and a plurality of control rods each having control blades are arranged between the channel boxes, long blade control rods each having control rod blades which extend in 4 directions latitudinally are arranged between channel boxes on diagonals of square bundle regions each formed by a plurality of fuel assemblies, and short blade control rods each having a control rod blade length in a lititudinal direction of about one half of the width of a square bundle region are arranged between the channel boxes in the center of each of the square bundle regions.
According to the present invention, in the long blade control rod, a region in which the long blade control rod covers the fuel assembles increases by an increment in the blade latitudinal length, whereby the control rod worth as a single control rod increases. Accordingly, since the control rod itself becomes large in size, there is left a problem of securing a reactor shutdown margin in a case where one control rod can not be inserted.
To solve the problem, change in the reactivity when one of the long blade control rods is pulled out is analytically obtained. FIGS. 3A and 3B each show an uncontrollable region of fuel assemblies when one 6, 4 of the control rods was pulled out, in comparison with the conventional lattice. In FIGS. 3A, 3B, the control rod 6, 4 which was pulled out in this example corresponds to a stuck control rod (failed to be inserted) or a control rod pulled out by mistake. These cases are substantially the same as each other from a viewpoint of reactor shutdown margin
Since the large-sized lattice fuel assembly of the present invention corresponds to four of the conventional fuel assemblies and the large-sized fuel assembly is divided into 4 blocks (hereunder, referred to as mini bundles), that is, since a square bundle region defined by latituinal long control rod blades is constituted of 4 of the large-sized fuel assemblies and each large-sized fuel assembly is divided into the four mini-bundles, an uncontrollable region by the control rods is shown on the mini bundles by half tone expression.
Numbers 1, 2, 3, 4 given on each mini-bundle express fuel at the first cycle, the second cycle, the third cycle and the fourth cycle after loading. An example of a fuel loading pattern is shown by the numbers 1-4. This pattern is an example of a practical fuel arrangement in view of the following point.
Rearrangement of the mini-bundles forming a fuel assembly is not conducted an avoid to increase in labor for fuel exchange; and concentration of bundles of the same cycle (under a sever condition) around one control rod should be avoided.
In the conventional lattice, an uncontrollable region is a rhombic region including the control rod, The present invention, however, includes 4 small rhombic regions (shown by half-tone) other than the above-mentioned rhombic region, and the shutdown margin seems to be reduced. However, since the small rhombic regions are surrounded by lititudinal short blade control rods 7 adjacent thereto and latitudinal long blade control rods 6, it is expected that the regions are influenced by those short control rod blades and long control rod blades and the effect of reduction of reactor shutdown margin is small in the regions. According to the analytic result, compared with the conventional lattice, a reduction amount of the reactor shutdown margin in the present invention is 1 %xcex94k or less, the reactor shutdown margin is almost the same as the conventional lattice, and it is found that the reactor shutdown margin can be secured.
Therefore, use of the latitudinal long blade control rod enables increase in control rod worth as a single rod, so that the number of the control rods and the number of control rod driving devices can be reduced largely by the number corresponding to an increment of the control rod worth and a cost can be reduced. Further, the control system can be simplified by reduction of both the number of the control rods and the number of the control rod driving devices.